PROJECT SUMMARY Men who have sex with men (MSM) are disproportionately affected by HIV, and more than 70% of HIV infections among youth (? 24 years old) occur among young MSM (YMSM). US Food and Drug Administration approved oral fluid rapid HIV self-testing (HIVST) devices provide a critical route for reducing barriers to HIV testing; however, uptake of HIVST among YMSM has been slow to date and no known studies have explored barriers and facilitators to HIVST among YMSM using rigorous mixed-methods procedures. YMSM who engage in HIVST are also good candidates for HIV pre-exposure prophylaxis (PrEP), but PrEP uptake among YMSM has been minimal to date. The Information-Motivation-Behavioral Skills (IMB) model is an empirically supported framework for HIV prevention, which can be applied to the barriers and facilitators related to HIVST and PrEP uptake. An IMB-guided mHealth intervention is a promising strategy to increase HIVST and PrEP uptake, offering participant confidentiality and potential scaling of an efficient intervention. Despite the promise of this type of theoretically-guided intervention, little is known about the independent effects of each IMB model construct on HIVST and PrEP uptake. To support intervention development, we will conduct mixed-methods formative research to explore IMB-related barriers and facilitators to HIVST and PrEP uptake among YMSM using data collected via online focus groups (n = 12; 96-144 individuals) with YMSM nationwide and a national survey of YMSM (n = 500; i.e., Aim 1). We will then pilot an IMB-factorial mHealth intervention to increase HIVST and post-test PrEP uptake tailored for YMSM, assessing feasibility, acceptability, and preliminary impact at 3-month follow-up (i.e., Aim 2). The intervention will be reviewed by an online youth advisory board and beta-tested with 12 YMSM. We will then pilot the clinical trial with 120 YMSM randomized to one of four groups using a factorial randomized experimental design, allowing us to determine which IMB model construct(s) are most important to increase HIVST and PrEP uptake. The four arms of this randomized controlled trial will be information-only (I), information + motivation (IM), information + behavioral skills (IB), and full IMB model (IMB). Dried blood spot sampling will be used to measure PrEP uptake and adherence. The training plan of this K01 award is designed to facilitate Dr. Steven A. John's transition to an independent investigator. Specifically, Dr. John will: (1) develop skills in mixed-methods research including advanced training in qualitative research methods, (2) build content expertise in mHealth interventions for HIV prevention, and (3) gain expertise in translating formative work into efficacy trials with supplemental training in quantitative analysis and implementation science. Additional training in responsible and ethical conduct in research and a focus on career enhancement and professional development will accompany the three primary training goals. The training plan includes didactic coursework, independent study, training institutes, academic conferences, manuscript preparation, grant writing, and other research activities.